Patterned object recognition systems are becoming common in industrial and commercial settings and have a variety of uses. For example, such systems can be used in scanners for the scanning of text, drawings, and photographs. Recently, manufacturers have been attempting to reduce costs associated with pattern recognition systems to make them more viable for consumer use. One such consumer application for pattern recognition systems includes fingerprint acquisition and recognition. Such a system is useful, for example, to enhance computer security by reading a potential user's fingerprint to compare with the fingerprints of users authorized to use the computer or access certain files or functions of the computer. Such a system could, for example, take the place of a security system that uses a login name and password.
The first thing such a fingerprint recognition system, or any pattern recognition system, must be able to do is to accurately acquire the fingerprint, or other pattern, for analysis. A number of mechanisms exist for such acquisition of pattern data. For example, U.S. Pat. Nos. 3,975,711; 4,681,435; 5,051,576; 5,177,435 and 5,233,404 all disclose apparatuses for acquiring an image of a patterned object.
FIG. 1 shows a schematic diagram of one such prior art optical fingerprint capturing and recognition system. In FIG. 1, an optical recognition system 108 includes a light source 112, an optical triangular prism 110, a lens assembly 114, an image sensor 116, and a storage and processing unit 125. The prism 110 includes an imaging surface 118, a light receiving surface 120, and a viewing surface 122. Imaging surface 118 is the surface against which a patterned object, such as a fingerprint, is placed for imaging. The light source 112, which may, for example, be a light emitting diode (LED), is placed adjacent to light receiving surface 120 and generates incident light 124 that is transmitted to the optical prism 110. The optical prism 110 is an isosceles right triangle, with the angle opposite the imaging surface 118 being approximately 90 degrees and the other two "base" angles (that is, the two angles of an isosceles prism that are equal) each being approximately 45 degrees.
Generally, incident light 124 strikes imaging surface 118 at an angle 126 with the incident surface normal line 115. Angle 126 is greater than the critical angle 128. In general, a critical angle is measured between an incident light ray and a normal line to a surface. If incident light strikes a surface at an angle greater than the critical angle, the incident light will undergo total internal reflection off the surface, if the incident light strikes the surface at an angle less than the critical angle, the incident light will substantially pass through the surface. Accordingly, critical angle 128 is the angle with the normal line to the imaging surface 118 above which incident light will totally internally reflect from imaging surface 118 and pass out of prism 110 as reflected light 130 through viewing surface 122.
Reflected light 130 passes through lens assembly 114 located adjacent to viewing surface 122. Lens assembly 114 may contain one or more optical lenses. Thereafter, light from lens assembly 114 is captured by image sensor 116. Image sensor 116, which may, for example, be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, captures optical light images and converts them to electrical signals. Such image sensors are well known to those skilled in the art. The electrical signals are then transmitted to the storage and processing unit 125.
Storage and processing unit 125 may include a memory unit, a processor and an analog to digital converter (not shown). The analog to digital converter converts the analog electrical signals from the image sensor 116 into digital data. The memory is used to store the digital data and algorithms for comparing a captured fingerprint image with a stored fingerprint image. The processor compares the captured digital data with data previously stored in memory based on an algorithm for comparing such data. The processor may also analyze the captured digital data for purposes different from comparison with stored data. Such storage and processing units are known to those skilled in the art and can include standard personal computers equipped with appropriate software. Algorithms for processing and comparison of image data are disclosed, for example, in U.S. Pat. Ser. Nos. 4,135,147 and 4,668,995 each of which is incorporated in its entirety by reference.
When a fingerprint is placed on the optical prism's imaging surface 118, ridges 111 of the fingerprint contact imaging surface 118, and valleys 109 of the fingerprint remain out of contact with imaging surface 118. Thus, in fingerprint valleys 109 incident light 124 entering optical prism 110 from light source 112 undergoes total internal reflection at imaging surface 118 if the incidence angle of the incoming light exceeds the critical angle of the optical prism 110. However, at ridges 111 of a fingerprint some of incident light 124 is absorbed and scattered off the fingerprint ridge. As used herein, the term "scattered" indicates light which, after striking an irregular surface, is radiated or irregularly reflected off the irregular surface in multiple directions.
As a result of this scattering and/or absorption, there is less than total internal reflection of incident light 124 at fingerprint ridges 111. Thus, the intensity of reflected light 130 leaving prism 110 from the valleys 109 of a fingerprint is of greater intensity than reflected light 130 leaving prism 110 from ridges 111. The lower intensity reflected light 130 from ridges 111 translate into darker regions to indicate the presence of an object at the point of incidence between the light beam and the fingerprinting surface. Conversely, higher intensity reflected light 130, such as that which undergoes total internal reflection, translates into brighter regions to indicate the absence of an object at the point of incidence between the incident light 124 and the imaging surface 118. This allows distinguishing the darker fingerprint ridges 111 from the relatively brighter fingerprint valleys 109. Because absorption of incident light at fingerprint ridges 111 is primarily responsible for creating a fingerprint image, system 108 is referred to as an "absorption" imaging system.
The above described system allows capturing an optical fingerprint image and processing the electrical representation of the optical fingerprint image. However, in regions of fingerprint ridges 111, incident light 124 still undergoes some total internal reflection and some scattering in a direction parallel to reflected light 130. Thus, the difference in intensity between reflected light 130 from fingerprint valleys 109 and fingerprint ridges 111 can be relatively low. That is, the contrast between fingerprint ridges 111 and valleys 109 in the fingerprint image can be relatively low. This can make image acquisition, processing, and comparison relatively difficult.
Additionally, in optical recognition system such as optical recognition system 108 it can be desirable that the diameter of the first lens in lens assembly 114 be smaller than the image of a fingerprint on viewing surface 122. This both allows optical recognition system 108 to be relatively small and can be less expensive to manufacture.
However, as shown in FIG. 2, in an absorption type system such as system 108, if the diameter of the first lens of lens assembly 114 is smaller than the fingerprint on imaging surface 118, then the lens assembly 114 must generally be placed relatively far from viewing surface 122. This allows the image of a fingerprint captured by system 108 to be relatively sharp all the way to the edges of the fingerprint image. That is, if lens assembly 114 is placed too close to viewing surface 122, the edges of a fingerprint image could be lost or distorted near the edges of the image. This is because in an absorption system such as system 108, the light rays which generate the image of the fingerprint must be substantially parallel for the image to be in focus. And, if the first lens in lens assembly 114 is smaller than the fingerprint in imaging surface 118, then the light rays from the edges of the fingerprint image that are parallel to light rays from areas closer to the center of a fingerprint image may not be able to enter lens assembly 114. This can cause the edges of a fingerprint image to be out of focus or lost.
Thus, as shown in FIG. 2, if the lens assembly for optical recognition system 108 were placed where lens assembly 114' is shown (in phantom), then substantially parallel rays of reflected light 130 and 130' would not enter lens assembly 114'. For this reason, system 108 would not produce a sharp image of a fingerprint placed on imaging surface 118 at points A and B if the lens assembly were placed at the location of lens assembly 114'.
Thus, as shown in FIG. 2, in an absorption system, the reduction in size gained by manufacturing a relatively small first lens of lens assembly 114 can be lost because lens assembly 114 must be placed at a relatively large distance from viewing surface 122 in order to capture the entire fingerprint image using light rays that are substantially parallel. For this reason, making optical recognition system 108 relatively compact can be problematic. Additionally, a relatively large distance between viewing surface 122 and lens assembly 114 can cause loss of contrast in the fingerprint image due to light interference.
Further, when the first lens in lens assembly 114 is smaller than an image of a fingerprint at viewing surface 122, a phenomenon known as trapezoidal distortion can occur in optical recognition system 108. Trapezoidal distortion in an imaging system has the effect of making the image of a square created by the system appear as a trapezoid.
FIG. 2 is a schematic illustration showing why trapezoidal distortion arises in optical recognition system 108. Incident light 124 from light source 112 enters prism 110 and reflects off imaging surface 118, imaging object AB. Reflected light 130 then passes out of viewing surface 122 and to lens assembly 114 at points A' and B' to form object A'B'. Viewing object AB through viewing surface 122, object AB would appear to be located at an "apparent image" object ab. Specifically, point A appears to be at point a, a distance aa' from viewing surface 122 and point B appears to be at point b, a distance bb' from viewing surface 122. The distance that an apparent image of an object appears from viewing surface 122 is given by the actual distance the object is from viewing surface 122 divided by the index of refraction n of prism 110. Specifically, the distance aa' is given by:
aa'=Aa'/n, PA1 bb'=Bb'/n.
where n is the index of refraction of prism 110. Similarly,
Trapezoidal distortion occurs when the light path length from the apparent image of an object to the lens plane 107 of lens assembly 114 is different for different parts of the imaged object and the object lens of the lens assembly 114 is smaller than the image of the fingerprint through viewing surface 122. Specifically, trapezoidal distortion occurs in system 108 because the distance aA' is longer than the distance bB' and lens assembly 114 has a smaller diameter than the distance a'b' on viewing surface 122.
Another consequence of distance aA' being larger than distance bB' is that an image of an object which is sharply focused at each part of the image can be difficult to obtain. More generally, whenever the light path length from the apparent image of an object to the lens plane, and ultimately image sensor, of a lens assembly is different for different parts of the imaged object, parts of the image of the object at the lens plane may be in relatively sharp focus and parts of the image may be out of focus.
To correct both the problems of trapezoidal distortion and having a portion of an image of an object which is out of focus, prior art manufacturers have tilted the lens plane 107 of lens assembly 114 and image sensor 116 to increase the distance bB' and decrease the distance aA' to a point where the two distances are approximately equal. However, it is a property of an isosceles right prism (that is, a triangular prism in which the base angles measure approximately 45 degrees and the non-base angle, or apex angle, measures approximately 90 degrees), that reflected light 130 exits prism 110 substantially normal to viewing surface 122. That is, no refraction of reflected light 130 occurs as it exits viewing surface 122. Further, generally, the larger the angle of incidence on a surface of a transparent object, the greater the portion of incident light that is reflected from the surface. Thus, while tilting lens assembly 114 and the sensor can reduce trapezoidal distortion and increase image sharpness, it also causes greater reflection of reflected light 130 off the surface of lens assembly 114, and the surface of image sensor 116, because reflected light 130 strikes lens assembly 114 at a greater angle of incidence. This reduces the intensity of light entering image sensor 116, making image processing and comparison more difficult.
Additionally, the relative placement of light source 112 and lens assembly 114 make it possible for stray light 113 emitted by light source 112 to enter lens assembly 114. This can generate additional background "noise" light which can further reduce the quality of a captured image and make image processing more difficult.
To overcome some of the difficulties associated with the type of absorption image acquisition system described above, acquisition systems have been designed which are based primarily on "scattering" mechanisms rather than absorption mechanisms. One such acquisition system is disclosed by U.S. Pat. No. 5,233,404 issued to J. Lougheed et al. on Aug. 3, 1993 (Lougheed et al.). FIG. 3 is a schematic diagram illustrating the image acquisition portion of the apparatus disclosed by Lougheed et al. As shown in FIG. 3, a prior art image acquisition system 208 includes a trapezoidal prism 210, a light source 212, a lens assembly 214 and an image sensor 216. The trapezoidal prism 210 includes at least an imaging surface 218, a light receiving surface 220, and a viewing surface 222.
The imaging surface 218 is the surface against which an object to be imaged, such as a fingerprint, is placed. The light source 212 is located adjacent to and facing the light receiving surface 220 which is substantially parallel to imaging surface 218. Thus, incident light 224 emitted by light source 212 projects light through prism 210 and onto imaging surface 218 at an angle which is generally less than the critical angle 228 of prism 210. Therefore, in the valleys 209 of a fingerprint placed against imaging surface 218 where the fingerprint is not in contact with imaging surface, total internal reflection does not occur and incident light 224 passes through imaging surface 218. At points where fingerprint ridges 211 are in contact with imaging surface 218, incident light 224 strikes the fingerprint ridge to generate scattered (or equivalently, irregularly reflected) light 230. Scattered light 230 propagates back into prism 210 in substantially all directions including the direction of lens assembly 214, located adjacent to viewing surface 222. Scattered light passes through viewing surface 222 and into lens assembly 214 to be detected by image sensor 216, which, as above, can be a CCD, CMOS or other type of detector.
In the region of a fingerprint valley 209, incident light 224 passes through imaging surface 218. And, in the area of a fingerprint ridge 211, incident light 224 scatters off imaging surface 218 to be picked up by lens assembly 214 and image sensor 216. Accordingly, the image of the fingerprint is relatively bright at fingerprint ridges 211 and relatively dark at fingerprint valleys 209. Because scattered light 230 is picked up by the image sensor 216, this type of system is referred to as a "scattering" system.
The difference in intensity, or ratio of intensity, between the ridges and valleys in a fingerprint image created by such a scattering system can be greater than the difference in intensity, or ratio of intensity, between the ridges and valleys of a fingerprint image created in an absorption system as shown in FIG. 1. As a result, the fingerprint image created by such a scattering system can display higher contrast between fingerprint ridges and valleys than an image created by an absorption system. Thus, the image can be more accurately acquired by the image sensor 216. This can reduce errors in subsequent fingerprint comparisons performed by the system.
Additionally, it is a property of a scattering system that the rays of light which enter lens assembly 214 to produce an image of a fingerprint in a scattering system do not need to be parallel to produce a sharp image. Thus, if the first lens in lens assembly 214 is smaller than the image of the fingerprint in viewing surface 222, lens assembly 214 can still be placed relatively close to viewing surface 222 without loss of image sharpness near the edges of the image.
However, a trapezoidal prism such as prism 210 can be more expensive to manufacture than a triangular prism such as prism 110, shown in FIG. 1. This is because, among other reasons, there is an extra surface to polish. This can increase the price of an imaging system such as imaging system 208, making it less viable for consumer use.
Additionally, because of differences in scattered light path lengths from different portions of the apparent image of the fingerprint in prism 210 to lens assembly 214, image acquisition system 208 can cause portions of a fingerprint image to be out of focus in a manner similar to that of optical recognition system 108. Additionally, though not shown in FIG. 3, if the first lens in lens assembly 214 of image acquisition system 208 is smaller than a fingerprint image on viewing surface 222 the differences in scattered light path lengths from different portions of the apparent image of the fingerprint in prism 210 to lens assembly 214 and image sensor 216 can also cause trapezoidal distortion.
As the above discussion makes clear, there is a need for improved image acquisition apparatus for use with patterned object recognition systems. Specifically, an image acquisition apparatus which produces an image having reduced or substantially eliminated trapezoidal distortion would be desirable. Additionally, an image acquisition system which generates an image in which substantially the entire image is in focus is also desirable. The image acquisition system should also be relatively compact and inexpensive to manufacture.